1. Field of the Invention
The present invention relates to an image forming apparatus and, more specifically, to a recording paper conveying device, a document feeding device and an image forming apparatus provided with these devices, preventing unnecessary deflection of recording paper in front of a registration roller pair and thereby enabling reduction in size of products.
2. Description of the Background Art
In an image forming apparatus such as a copying machine, a sheet of recording paper is drawn out from a paper feed tray and fed to an image forming unit, and an image is formed (printed) on a surface of the recording paper. By way of example, if electrophotography is utilized, a toner image carried on a photoreceptor drum or on a transfer body is transferred to a fed sheet of recording paper to obtain an image. The sheet of recording paper is drawn out from the paper feed tray by means of a roller, pinched by opposing rollers arranged along a paper feed path, and fed by the rotation of rollers. When a sheet of recording paper is drawn out from the paper feed tray, leading edge of the sheet may possibly be drawn out not in an orientation perpendicular to the drawing direction, but rotated to some extent. During feeding of the recording paper, it is also possible that the sheet is conveyed with its leading edge not kept perpendicular to the feeding direction. If the sheet of recording paper is fed askew to the transfer unit, an image will be formed askew with respect to the longitudinal or lateral side of the recording paper.
Similar problem may occur when a document is fed to a scanning position using an automatic document feeder (ADF) in an image forming apparatus. If the document is fed askew, the document will be scanned askew.
Therefore, conventionally, skew correction of recording paper is done immediately preceding a registration roller pair (upstream side in the paper feed direction) for supplying the recording paper to the transfer unit. Specifically, a conveyor roller pair (in the following, also referred to as a pre-registration roller pair) arranged immediately preceding the registration roller pair feeds a sheet of recording paper to the registration roller pair of which rotation is suspended, leading edge of the sheet is brought into contact with or abuts the registration roller pair so that the sheet is deflected, and then rotation of the registration roller pair is started, to convey the sheet of recording paper. By way of example, the velocities of registration roller pair and pre-registration roller pair are controlled as shown in FIG. 1. The upper part of the graph shows time-change of the velocity Vr of registration roller pair, and the lower part of the graph shows time-change of the velocity Vp of pre-registration roller pair. By the pre-registration roller pair, the sheet of recording paper is fed to just in front of the registration roller pair, and then, the pre-registration roller pair is temporarily suspended (time t0). By the time the leading edge of the sheet reaches a prescribed position (just in front of the registration roller pair) and the pre-registration roller pair stops rotation, rotation of the registration roller pair has been suspended. At time t1, the pre-registration roller pair starts rotation (while the registration roller pair is stopped), at time t2, the leading edge of the sheet abuts the registration roller pair, and by the time t3, the sheet of recording paper is deflected by a prescribed amount. Thereafter, rotation of the registration roller pair is started, so that the sheet of recording paper is conveyed. In this manner, even if the sheet of recording paper has been fed askew toward the registration roller pair, the sheet of recording paper can be supplied to the transfer unit with the skew corrected. It is noted here that in the time period between t3 to t6, the pre-registration roller pair rotates at a constant velocity of Vc, while the velocity of registration roller Vr is lower than the velocity Vc of pre-registration roller pair. Consequently, the sheet of recording paper is much deflected. The length that corresponds to the area of hatched portion S0 in FIG. 1 represents excess deflection.
As a solution to this problem, Japanese Patent Laying-Open No. 2010-111472 (hereinafter referred to as '472 Reference) discloses a technique of deflecting a sheet (of recording paper) by a prescribed amount by the rotation of pre-registration roller pair, and thereafter accelerating the registration roller pair and the pre-registration roller pair simultaneously and at the same rate of acceleration to attain a constant velocity, as shown in FIG. 2. This prevents excess deflection of the sheet.
In '472 Reference, however, the velocity of rotation of pre-registration roller pair to cause the prescribed amount of deflection is a low velocity (100 mm/s) that corresponds to a self-start frequency at which a stepping motor driving the pre-registration roller pair can start rotation, and the pre-registration roller pair is once stopped. Therefore, it takes time to form an image on the sheet of recording paper, and performance of the image forming apparatus cannot be much enhanced. This poses a significant problem in a high-speed apparatus.